1. Field of the Invention
The present invention relates to a high average power magnetic modulator for metal vapor lasers and the like.
2. Description of the Related Art
Many different types of lasers are energized by pulsed electrical discharges. Where higher average output power is desired from a pulsed laser system, more power per pulse, shorter pulse lengths and less time between pulses is generally required. Extensive experience with thyratron driven capacitor inversion circuits for driving moderate power metal vapor lasers such as copper vapor lasers (about 10 kW input) teaches that in such circuits the thyratrons do not exhibit the lifetime required for the new higher power output copper vapor lasers. The operating constraints that moderate power modulators impose on a thyratron with regard to di/dt, peak current amplitudes, and repetition rates often cause premature thyratron failure due to cathode depletion, gas depletion, and anode erosion; a significant increase in the power switched by the thyratron would only exacerbate this problem. An examination of other known types of high voltage modulator switches, technologies, and circuit topologies has not revealed a simple, inexpensive, or proven alternative. The search for longer lifetimes has led to the use of magnetic compression circuits even though they are a more complex and more expensive technological approach as compared with thyratron switched capacitor inversion circuits. While the use of magnetic compression circuits reduces the electrical stress on the thyratrons (often used as the primary switching device) and thereby increases the thyratrons"" operating lifetime, experience with such systems still indicates insufficient thyratron lifetime. Although thyratrons are designed as high peak power, high repetition rate commutation devices, thyratrons by their construction are plasma devices and have infinite lifetimes due to gas cleanup, cathode depletion and filament lifetime limits. This leads to the conclusion that other types of commutation switches must be used in conjunction with magnetic compression circuits to achieve the required power levels, reliabilities and lifetimes.
In an attempt to overcome the short device lifetimes and other limitations of thyratron driven modulators, solid state devices such as thyristors and Silicon Controlled Rectifiers (SCRs) have been used in the past as commutators in magnetic compression circuits for driving high power lasers. Such solid state devices have a potentially very long lifetime (more than tens of thousands of hours) if their performance ratings are not exceeded. However, solid state switches are relatively slow devices (conduction times are usually in the tens of microseconds and longer as compared with thyratrons that can have conduction times measured in tens of nanoseconds and longer) designed for high average power but not capable of switching high peak power. Accordingly, they must be coupled with a magnetic compression circuit in order to drive high peak power loads such as a gas discharge load.
Solid state devices are low voltage devices; most of the high average power devices designed for short conduction times (inverter grade devices) have maximum voltage ratings of less than 1500 volts. Consequently, to drive high voltage loads, solid state devices have to be stacked in series to increase the overall switching voltage and/or a step-up pulse transformer must be incorporated into the modulator circuit. If devices are not stacked in series, several devices operating in parallel are usually required and a very high step-up ratio, pulse transformer is required. Either series operation or parallel operation of solid state devices have both advantages and disadvantages.
A series stack can be designed to be equivalent to a thyratron in voltage rating and can replace the thryatron in a magnetic compression circuit if the switch conduction time is long enough; thyristors and SCRs do not switch or turn on as fast as thyratrons and cannot handle large rates of change in current (the maximum di/dt for most available devices is less than 1000 amperes per microsecond). The advantages of a series stack include: additional devices can be used to provide redundancy and increase the operating lifetime (one or more devices can fail (short) but the stack and modulator can continue operating); at high voltages the peak current requirements through switches and other components at the modulator input are reduced; and a pulse transformer, if required, will have a lower step-up ratio and low leakage inductance (required for an efficient pulse transformer) will not be difficult to attain. The disadvantages include: a series stack is expensive because, with a safety factor, twenty or more devices (at a 1200 volt rating) are required to make up a 20 kV thyratron replacement; voltage grading and overvoltage protection devices must be placed across every device; devices must be reasonably matched as to turn-on times to insure simultaneous or near simultaneous stack turn-on time (although external devices such as a magnetic assist can alleviate this problem); and individual trigger circuits with high voltage isolation are required for each device.
A switching circuit consisting of parallel solid state devices limits the input voltage of the modulator to the maximum operating voltage of the devices. Advantages of parallel operation include: the number of switching devices decreases as the energy transfer time increases; using few devices reduces the cost and volume; and the devices and their trigger circuits only need circuit isolation for 1 kV. Disadvantages include: a single device failure (short) results in modulator failure; the modulator circuit must be designed to insure current sharing between devices; operation at lower voltages implies high peak and rms currents so capacitors and conductors are required to have small series resistances in order to keep power losses small; and the required high step-up ratio pulse transformer (1:60 to 1:80) with very low leakage inductance can be inexpensive and difficult to manufacture.
Magnetic compression circuits are well-known in the art for having the capability of generating high peak power, short time duration voltage pulses by time compression of energy. Being composed of passive circuit elements (capacitors and non-linear inductors) they are very robust and can be very reliable. In application, magnetic compression circuits usually serve as an interface between a controllable switching device and a power load that usually requires high voltage, high peak power, short time duration, and often high repetition rate pulses. The controllable switching device is usually incapable of driving the load directly with any reasonable reliability or lifetime. The complexity and cost of magnetic compression circuits usually restrict their use to high average power, high repetition rate systems; metal vapor laser systems have these requirements and have utilized magnetic compression circuits. Such pulsed lasers are utilized in many applications such as medical diagnostics, laser isotope separation of an atomic vapor (known as an AVLIS (Atomic Vapor Laser Isotope Separation) process), and many other applications.
The basic principle underlying magnetic pulse compression operation involves a saturable inductor, often referred to as a magnetic switch, which consists of a winding around a saturating magnetic core. In operation, the inductance of the magnetic switch will change from a large value (unsaturated core) to a small value (saturated core) when a voltage is applied across the switch for a specified length of time. The gain of a magnetic switch is defined as the ratio of two time periods; the time that the magnetic switch can hold-off an applied voltage (prior to core saturation) and the time required to transfer energy through the switch (after core saturation). A typical magnetic switch has a gain of between 3 to 10. The resonant circuit consisting of the saturable inductor and two capacitors of approximately equal value that are usually connected between ground and the input and output of saturable inductor is commonly referred to as a stage of compression, a compression stage, or simply, a stage. Typically, the output capacitor of a stage of compression is the input capacitor to the next compression stage. The compression stages are cascaded and energy is coupled faster and faster from one stage to the next (the overall gain of a multistage magnetic modulator is the algebraic product of the gains of the individual stages). Referring to FIG. 1, a typical modulator uses a plurality of stages as shown in order to achieve an effective change in impedance much larger than can be obtained from a single stage. In a conventional magnetic modulator as shown in FIG. 1, capacitor C1 is charged through an inductor L0. When C1 is fully or almost fully charged, saturable inductor L1 saturates. L1 is chosen to have a saturated inductance much less than L0. Once L1 saturates, capacitor C2 will begin to charge from C1 through L1, but because the saturated inductance of L1 is much less than that of saturable inductor L0, capacitor C2 charges much more rapidly than C1 was initially charged. In a lossless circuit, all of the energy in C1 is transferred to C2 provided C1 is equal in value to C2. The process continues through the successive stages until capacitor Cn discharges into the load. Before the pulse compression sequence can be repeated, the magnetic cores of the modulator must be reset. The maximum pulse repetition rate is constrained by the time required for the pulse energy to propagate through the modulator, the recovery time required by the main switch, and the time required to reset the saturable inductors.
Conventional magnetic pulse compression circuits transfer power in both directions. They not only act to increase the pulse frequency in the forward direction thereby providing temporal compression of the pulse, but also decrease the pulse frequency of the voltage pulse as it cascades back through the compression stages in the reverse direction. The energy which reflects back from an impedance mismatch at the laser load (the typical case) can propagate up the chain to the commutator switch.
A disadvantage associated with conventional magnetic compression circuits is, that because they are resonant circuits, residual energy left behind in the circuit after the main drive pulse has passed will lead to oscillations. These low level oscillations may continue on for durations several orders of magnitude longer than the initial compression time and may cause problems in resetting the cores of the saturable reactors to a repeatable reset position, thus causing variations in the saturation time from pulse-to-pulse. This pulse-to-pulse variation produces undesirable or unacceptable pulse-to-pulse timing jitter at the load.
Besides pulse compression, laser modulators are usually called upon to perform voltage step-up as well. In a laser modulator, the pulse transformer usually performs the key function of voltage step-up for impedance matching and/or voltage isolation. Various design constraints must be taken into account when building a high frequency transformer. In particular, leakage inductance, which is inversely proportional to how well the primary and secondary winding are coupled to each other, slows down the risetime of the pulse through the transformer and is detrimental to the modulator efficiency and performance. In a conventional pulse transformer, low leakage inductance is achieved by interleaving the primary and when the goal is also to achieve large voltage differences between the primary and secondary windings. This approach is made more complicated when the goal is also to achieve large voltage differences between the primary and secondary winding. This can result in large and complex structures.
Driving metal vapor lasers such as copper vapor lasers is difficult because the lasing action is a direct result of an electrical discharge through a low pressure buffer gas and presents a highly nonlinear, time-varying load to the modulator. The discharge resistance in such a laser may change several orders of magnitude over the duration of a short (approximately 30-100 nanosecond) drive pulse. In addition, the laser head inductance is large in comparison with the RMS discharge impedance and thus a large fraction of the drive energy ends up stored in this inductance. As a result, high frequency, high voltage oscillations appear at the output of the laser modulator and attempt to couple high frequency energy back into the modulator rather than into the collapsing resistance of the discharge.
In summary there is a need for a long life (1011-1012 shots) and high reliability (capable of greater than 5000 hours between failures) high power laser modulator. Thyratron driven circuits typically are incapable of meeting these critical performance objectives because they develop discharge instabilities which lead to localized arc formation, electrode erosion, gas depletion and premature device failure. In comparison, solid state devices are capable of very long lifetimes and have no known wearout mechanisms. There is thus a need for an improved solid state switched modulator for efficiently driving high average power metal vapor lasers at high repetition rates and with increased reliability.
It is an object of the present invention to provide an improved magnetic modulator for high average input power (greater than 30 kW) metal vapor lasers. The magnetic modulator system described herein and shown in schematic form in FIG. 2 utilizes parallel solid state commutation devices, a novel high step-up ratio fractional turn transformer, and three stages of magnetic compression. Other components and sub-circuits critical to its operation are also described.
In accordance with one aspect of the invention, the energy transfer circuit which provides the drive to the first stage of the pulse compression circuits is segmented into a plurality of parallel SCRs or other gate controlled solid state switches, each device connected electrically in common at the anode junction. In addition the cathode of each solid state device is connected to its own saturable inductor commonly referred to as a magnetic assist. To force the saturable inductors to saturate simultaneously, thereby allowing simultaneous current flow in each of the SCRs, all the saturable inductor windings are wound on the same magnetic core. The hold-off time provided by the magnetic assist enables each SCR to turn on more completely prior to current build up which occurs when the magnetic assist saturates, thus reducing the turn-on energy losses in the SCRs. The inductance of the saturable inductor also forces the SCRs to share current, thus preventing damage to an individual SCR due to overcurrent. The magnetic assist is in essence a magnetic switch with a gain of typically 0.2 to 0.3; i.e., the hold-off time of the magnetic assist is less than its energy transfer time. The magnetic assist may be connected to either the cathode junction as shown by the circuit topology in FIG. 2 or in the anode circuit (in which case the cathodes are electrically connected in common), as preferred by the designer or most beneficial for circuit implementation. The terms xe2x80x9cmagnetically assisted SCRxe2x80x9d and xe2x80x9cmagnetically assisted gate controlled SCRxe2x80x9d are intended to refer to both configurations set forth in this paragraph.
In accordance with another aspect of the present invention, a novel fractional turn pulse step-up transformer having a gain of 1:80 is utilized. The fractional turn transformer described herein is defined to have two or more single-turn transformer primary windings electrically connected in parallel and driven by a single source. In addition, a continuous and electrically isolated secondary winding(s) links electromagnetically and simultaneously to all of the parallel primary windings. The total step-up ratio is the product of the number of primary windings times the number of turns in the secondary winding. More than one secondary winding may be employed. This novel transformer has a very low leakage inductance which results in a more compact and efficient modulator structure.
The fractional turn transformer described herein has an extremely low leakage inductance because of the mechanical design which incorporates many parallel, low impedance, transmission line type feeds to the transformer primary windings which in turn completely surround the magnetic core material with wide, tightly coupled (i.e., small spacing between conductors and the magnetic core) conductors. The low leakage inductance reduces the total gain (and therefore the losses) required of the final stages of the modulator. The transformer itself exhibits very low copper (conductor) loss due to the wide primary conductors (which have a very small resistance even at the high frequencies where skin effects may dominate) and low core losses because the core undergoes only a small increment of its total available flux swing.
The transformer secondary winding consists of a plurality of stainless steel tubes connected with bus links at the ends to form one continuous winding. Stainless steel tubing is used as the secondary winding conductor because its high resistivity (compared with aluminum or copper) increases the winding resistance and reduces timing jitter by dampening oscillations which can cause variations in the starting point in flux density when the cores of the saturable inductors are being reset.
In accordance with another aspect of the present invention, a fractional turn step-up transformer having a novel electromechanical geometry is employed as to minimize flux losses and achieve high gain. The novel fractional turn, xe2x80x9c1/nxe2x80x9d, transformer comprises a plurality of xe2x80x9cnxe2x80x9d, primaries. A transformer primary winding consists of a pair of plates which attaches to the top-side and bottom-side of a mandrel on which is wound the magnetic core and a flux shield which is positioned on the outside diameter of the magnetic core. The top plate constitutes the input or drive side of the primary winding, while the return side of the primary winding consists of the bottom plate and the flux shield which is connected electrically to the bottom plate. The primary winding forms a tightly coupled, conducting loop around the magnetic core in that the plates, mandrel, and flux shield completely surround the core (usually toroidal) with small spacings on all sides. However, the input and return side of the primary winding must not touch. The plurality of transformer primary windings are stacked vertically on their common axis. Electrical connections to both the drive plates and the returns of the primary windings are made in a very low inductance manner such that each of the primary windings may be considered to be driven equally by the drive source. It is essential that the primary winding be physically very close to the magnetic core in order to minimize magnetic flux outside the magnetic core (stray flux). In addition, the secondary winding must couple (enclose) all the flux generated by current flow in the primary winding in order to achieve very low leakage inductance.
The secondary winding of the transformer is comprised of stainless steel tubes installed on a diameter smaller than that of the magnetic core (on transformer primary) and connected on each end with a bus link to adjacent stainless steel tubes installed on a diameter larger than that of the magnetic core. The rods extend vertically through the plurality of stacked primaries. The rods on the smaller diameter and larger diameter are connected thusly together such that they form a single electrically continuous winding which loops around the magnetic cores of all the primaries. To maintain the low leakage inductance, the tubes of the secondary winding pass through holes in the transformer mandrels, the drive and return plates, and the flux shields and are therefore completely isolated from the primary-windings. The secondary output voltage appears across the ends of the winding.
In accordance with another aspect of the present invention, energy from the reverse current due to the impedance mismatch at the laser load is recovered and utilized to recharge the intermediate storage capacitor. This novel process results in energy savings and thus increased overall operating efficiency.
In accordance with another aspect of the present invention and with respect to the pulsed voltage, the laser head is allowed to float with respect to earth ground. That is, opposite electrodes of the laser head are pulsed at, for example, +35 kV and xe2x88x9235 kV, respectively. Floating the output in this manner enables the size of the transformer to be reduced and also substantially reduces much of the electrical stress within the transformer.
In accordance with another aspect of the invention, increased efficiency is provided by grading the storage capacitors of each pulse compression stage so that they gradually increase in capacitance in the direction of the output. This provides maximum energy transfer from one stage of compression to the next. Without the grading of capacitors, energy losses due to the magnetic switch and resistive losses in conductors and capacitors will result in some energy being left on the magnetic switch""s input capacitor after the energy transfer has taken place.
Additional objects, advantages and novel features of the present invention will be set forth in part in the description which follows and in part become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages and features of the present invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.